The present invention relates to a suction apparatus for a printing press, which temporarily sucks and holds an object used for a printing operation and, more particularly, to a suction apparatus for a printing press, which sucks the trailing edge of a paper sheet through a suction port to the circumferential surface of a paper convey cylinder such as a reversing cylinder, an impression cylinder, or a transfer cylinder, thereby stably conveying the paper sheet.
In a conventional sheet reversing device for a double side printing operation in a sheet-fed printing press, which is disclosed in Japanese Utility Model Publication No. 1-36609, a plurality of suction heads for sucking and holding a paper sheet through an air suction force are provided to a reversing cylinder while being arranged in the axial direction in correspondence with the maximum paper size. With this arrangement, the trailing edge of a paper sheet wound on the surface of an impression cylinder is sucked by the suction heads of the reversing cylinder to separate the paper sheet from the surface of the impression cylinder. The suction heads sucking the trailing edge of the paper sheet move to the reversing grippers of the reversing cylinder and transfer the paper sheet. Thereafter, the suction operation of the suction heads is stopped to retreat the suction heads to a predetermined position, and the paper sheet is reversed.
In the above conventional suction apparatus, however, when a small-size paper sheet is used, external air is sucked through the suction heads at the two end portions not used for the suction operation. This results in a large decrease in suction force, so no small-size paper sheet can be sucked.
To solve this problem, shutter plates can be provided to the air passage holes of the suction heads at the two end portions. When a small-size paper sheet is to be sucked, the air passage holes are closed by the shutter plates to stop air suction through the suction heads at the two end portions. However, in an arrangement for only mechanically closing the air passage by the shutter plate, the air passage may be insufficiently closed, resulting in air leakage from a small gap between the shutter plate and the air passage hole. In case of such air leakage, the degree of vacuum in the suction heads at the central portion used for the paper suction operation is degraded to weaken the paper suction force. Therefore, a printing registration error is caused by a paper offset at the suction heads, or the paper sheet falls due to a suction error.